Recently, due to depletion of fossil fuels and deterioration of global environmental problems, there is growing interest in vehicles using electric energy such as hybrid vehicles and electric vehicles and practical use progresses. A vehicle using such electric energy often includes a power conversion device that steps down a voltage from a high voltage battery to supply power to a motor to drive wheels and supplies necessary power to a low voltage electric device. A switching power supply device is generally used in the power conversion device that supplies power to an electric device such as an air conditioner, a stereo system, and a controller of the vehicle. In addition, a bidirectional switching power supply capable of performing not only a step-down operation but also a step-up operation is required in the switching power supply device. This may be used to supply power from a low voltage battery to cause the high voltage battery to be operable, when the high voltage battery performs discharging.
Here, when the power conversion device converts the power, losses such as copper loss and fixed loss occur. The copper loss occurring in the switching power supply device is proportional to the square of a flowing current. For this reason, to improve conversion efficiency of the power conversion device, two or more switching power supply devices may be provided in parallel. Because an output current of each switching power supply can be halved by configuring the power conversion devices connected in parallel, the copper loss is reduced. As a result, the conversion efficiency of the power conversion device can be greatly improved.
Generally, in the switching power supply devices connected in parallel, an imbalance of a current occurs between the switching power supplies due to an element variation or a difference of wiring line lengths, in elements mounted on the switching power supplies. If the current imbalance occurs, the current concentrates on one side of the switching power supplies connected in parallel. If the current concentrates, the losses in the switching power supply devices become larger than losses in a normal state and a design with a margin is required for a current or heat dissipation design. Generally, because the individual switching power supplies connected in parallel are designed with the same design value, a design value with a margin is required for the switching power supply on which the current does not concentrate. Therefore, if the current imbalance occurs, a design value with a margin is required for a current or heat dissipation condition in the individual switching power supplies, which results in hindering reduction of sizes and costs of the switching power supplies connected in parallel.
Therefore, there is a method of independently controlling switching elements of each of the plurality of switching power supply devices connected in parallel, when the current imbalance occurs. When the current imbalance occurs, a current imbalance amount is detected and each switching power supply device is controlled independently, so that control to suppress a current imbalance difference is performed. By this control method, the current imbalance amount is controlled to be smaller, when the current imbalance occurs. Therefore, the current imbalance of the switching power supply devices connected in parallel is improved. For example, Japanese Patent Application Laid-Open No. 2010-124671 (PTL 1) is known as a power conversion device in which such control is performed during step-up in particular.